1. Field of the Invention
The invention relates to a method of removing an oxide layer within a metal interconnect to reduce the resistance thereof, and more particularly, to a method of removing a copper oxide layer within a via hole.
2. Description of the Related Art
As the integrated circuits are developed towards a direction of more and more compact and complex, multilevel metallization process is commonly used to connect different conductive layers. Two important techniques, those of forming a contact plug and a via, are often employed in a semiconductor. Contact plug is an embedded part of a metal layer for connecting electrodes of a metal-oxide semiconductor (MOS). A via plug is used for connecting different metal layers.
It is known that aluminum has a very low resistivity and a good adhesive property when formed on a silicon oxide layer. However, as shown in FIG. 1, while conducting, aluminum atom drifts towards a grain boundary due to an electric field. This is the so-called "electromigration" effect. If the electromigration is strong enough to induce electron drift, an open circuit is caused to affect device reliability as shown in FIG. 2. Therefore, copper, instead of using aluminum, is used to form a via plug.
FIG. 3A to FIG. 3C shows a conventional method of forming a via plug. In FIG. 3A, on the electrode of a chip, a copper layer 300, a silicon nitride layer 302, and a silicon oxide layer 304 are formed in sequence. The formations of the silicon nitride layer 302 and silicon oxide layer 304 are used to isolate the metal layers for connecting different electrodes, and therefore, to avoid a short circuit. In addition, the silicon nitride layer 302 formed between the copper layer 300 and the silicon oxide layer is for the purpose of avoiding the formation of copper oxide by directly connecting the copper layer 300 and the silicon oxide 304.
In FIG. 3B, a via hole 306 is formed to penetrate through the silicon oxide layer 304 and the silicon nitride 302 layer, so that the copper layer 300 is exposed within the via hole. As shown in the figure, a copper oxide layer 310 is easily formed by exposing the copper layer 300 to oxygen molecule 308 in the air.
In FIG. 3C, a first metal layer 312 is formed and fills the via hole 306. Another metal layer 314 is formed on the silicon oxide layer 304 and the metal layer 312.
When the metal layer connects to an external electric field, the existence of a copper oxide layer 310 between the copper layer 312 and the metal layer 312 increases the resistivity.